US2774021A

US2774021A - Electrical motor control apparatus

Info

Publication number: US2774021A
Application number: US449947A
Authority: US
Inventors: Robert J Ehret
Original assignee: Honeywell Inc
Current assignee: Honeywell Inc
Priority date: 1954-08-16
Filing date: 1954-08-16
Publication date: 1956-12-11
Anticipated expiration: 1973-12-11

Description

Dec. 11, 1956 R. J. EHRET ELECTRICAL MOTOR CONTROL APPARATUS Filed Aug. 16, 1954 JOmPZOO O2 INVENTOR. ROBERT J. EHRET HMT ATTORNEY.

United States Patent ELECTRICAL Moron CONTROL APPARATUS Robert J. Ehret, Philadelphia, Pa., assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware Application August 16, 1954, Serial No. 449,947

11 Claims. (Cl. S18-207) A general object of the present invention is to provide a new and improved motor control circuit. More specifically, the present invention is concerned with a control circuit for a two phase reversible induction motor of the type employed self-balancing measuring and control apparatus. in one form of such an apparatus, a measuring circuit unbalance is amplified by means of an electronic amplitier which is operative to impress upon the input of a motor control circuit a signal varying, in magnitude and phase, in accordance with the magnitude and direction of the imbalance. The motor control circuit, in turn, operates in accordance with the phase and magnitude of that signal to selectively energize the motor for rotation in the direction and to the extent necessary to rebalance the measuring circuit.

Accordingly, it is a specic object of the present invention to provide a new and improved motor control circuit employing a minimum of components and capable of efficiently driving a rebalancing motor in a measuring apparatus of the type described.

Motors, because of their inertia, have a tendency to coast or continue rotation after being deenergized. In a self-balancing measuring and control apparatus this tendency causes the rebalancing means to overshoot the point of balance which results in hunting. If the rebalancing motor is driven from a low impedance'source, however, this tendency of the motor to coast is greatly reduced.

lt is therefore another specific object of the present invention to provide an improved low impedance motor control circuit.

Transistors, unlike vacuum tubes, can be made to exhibit extremely high or extremely low impedances. These impedance characteristics of a transistor may be advantageously utilized in a motor control circuit to regulate not only its direction and rate of operation but also the rate at which the motor may be brought to a stop.

Accordingly, a further object of the present invention is to provide new and improved transistor motor control circuit in which a transistor shunts the motor control winding in such a manner as 'to provide a low impedance motor driving source and thus good motor damping.

Generally, it is desirable to keep the power dissipation in transistors relatively low. In order to obtain low dissipation the transistor may be biased so that the quiescent operating point is at either end of the load line, at a point of low power dissipation. In the present invention the motor control transistor is operated at a point or" high collector current and a low voltage. This method of operation is not generally used because ofthe low standby emciency resulting from the power loss in the transistor collector bias supply. However, this has been overcome by the provision of a special power source which operates in such a manner asto provide etlicient operation of the control circuit.

A still further object of the present invention is to operate a motor control transistor from` aconstant current source in such a manner that there is negligible power dissipated in the power supply.

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The various features of novelty which characterize this invention are pointed .out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its advantages, and specific objects attained with its use, reference should be had to the accompanying drawings and descriptive matter in which are illustrated and described preferred embodiments of this invention.

Of the drawings:

Fig. 1 is a circuit diagram of a preferred embodiment of the present invention;

Fig. 2 is a table showing voltages and currents present in di'terent parts for the circuits shown in Figs. l and 3 under various operating conditions;

Fig. 3 is a circuit diagram Vshowing the modification of the present invention as shown in Fig. 1.

Referring now to Fig. l, the numeral 1 represents a two phase reversible induction motor having a power winding 2, a control winding 3, and a-squirrel cage rotor 4. The power winding 2 is connected across a suitable source of alternating current, the conductorsLi and L2. Generally, the conductors L1 and L2 represent a source of ll() volts 60 C. P. S. alternating current. The motor control winding 3 is connected in parallel with the condenser 5 and the emitter-collector circuit of the transistor 6 across the output terminals 7 and 8 of the full wave bridge rectifier 9. The condenser 5 is selected, with respect to the motor control winding 3, so as to form therewith a substantially parallel resonant circuit. The condenser 5 provides an increase in motor power but it can be eliminated without substantially impairing the operation of the circuit.

The full wave bridge rectilier 9, the resistor 13, and the condense-r 14 comprise a constant current power supply generally designated vby the `numeral 10. The input terminals 1l and12 of the full wave bridge rectifier 9 are connected in series with the condenser 14 and the resistor '13 across the alternating current conductors L1 and L2. The resistance of the resistor 13 is comparatively small and it is included in this circuit to protect the transistor 6 from sudden surges of lineV current. If surge conditions are not prevalent the resistor 13 may be eliminated. The collector 15 of the transistor 6 is connected to the negative terminal 7 of the bridge rectilier 9 by means of the conductor 16. The emitter 17 of the transistor 6 is connected to the positive terminal 8 of the bridge rectifier 9 by means of the conductor 18.

The input of this circuit is across the

end terminals

21 and 22 o-f the primary winding 23 of the input .transformer 24. The input tnansformer 24 `has a secondary Winding 25 having one of its end terminals -connected to the emitter 17 of the transistor 6 and its other end terminal connected through Vthe condenser 26 to the ibase 27 lof ithe transistor 6. When so connected, the emitter t17 of the transistor 6 is common to both the input and output circuits and the transistor 6 is thus connected in the well known common emitter configura-tion. A bias-ing lmeans 31, including the resistor 32 and a sounce of direct cur- -rent Shown here as the battery 33, is yconnected in series across the base 27 and the emitter 17 of the transistor 6.

In considering the operation of the circuit shown Iin Fig. 1, the voltage across the conductors L1 Iand L2 will Ibe. considered as the reference voltage. The direction of the rotation :of the motor 1 depends upon the phase relationship `between the current in the control win-ding 3 'and .the current in the power winding 2. If the current :in lthe control winding leads theycurrent in the power winding by approximately degrees, the motor 1 will turn in one direction. If, on the other hand, the current in 'the control winding lags the current in "the power wind-v ing by approximately 90 degrees, they motor 1 will 'turn 4in the other direction.

`In operation, the power winding 2 is energized continuously Iby current from the conductors L1 and L2. Due to the relatively high inductance of the control winding 2, this current lags the voltage across the conductors L1 and L2 lby approximately 90 degrees. The magnitude and phrase of the current in the motor control winding 3 is controlled [by the transistor 6. Since the condenser I5 is chosen with respect to the inductance of the control winding 3 so as to form ytherewith a parallel resonant circuit, the parallel combination of the control winding 3, the condenser '5 and the emitter-collector circuit off the transistor 6 comprise a substantially resistive load -on the rectifier 9. The condenser 14, however, connected to series with the resistor 13 and the rectifier 9 across the conductors L1 and L2 is selected so as to have an extremely high capacitive reactance, at the frequency of the voltage across the conductors L1 Iand L2. -As a result, the pulses of full wave 4rectified current in lthe transistor 6 alternately lea-d and lag the voltage across the conductors L1 and L2 by approximately 90 degrees.

iAs shown in Fig. 2, in the absence of a control signal the bias supply 31 is operative to bias the transistor 6 so as to cause maximum current to flow -in the transistor emitter-collector circuit. The impedance of the emitter- -col-lector circuit of the transistor 6 under such conditions is so low compared to the impedance of the winding 3 as to prevent all 4but a negligible current from iiowing through the 'winding 3. Despite the Ilarge current ow :in transistor 6 very little real power is lost in the circuit while it is in -this standby condition. The major voltage drop -is -across the condenser y14, which represents a reactive power loss not a real power loss. Thus, by employing the transistor 6 to shunt the control winding 3 and supplying current to these elements from a current source of the type shown, it is possible to achieve eicient standby operation.

If a `control :signa-l of a suitable magnitude and leading or Ilagging the reference volta-ge by approximately 90 degrees is applied to the circuit [across the

input terminals

21 and 22, the effect of this ybias on the transistor 6 will be Vwholly or partly overcome during one half cycle `of the pulsating unidirectional current flowing in the transistor emitter-collector circ-uit. Such a control signal will cause the transistor collector voltage to rise, thereby aliowing current lfrom the bridge rectifier l9 to pass through the control winding 3 instead of through the transistor 6. Thus, the lbridge rectifier 9 and the capacitor 14 tend to become a constant current source Iapplied in shunt to the motor control winding 3 and the transistor 6.

Due to the inductance of the control winding 3, the current in that winding lags the voltage across it by approximately 90 degrees. I-f the cont-rol signal leads the reference voltage yby lapproximately 90 degrees, the voltage appearing across the emitter-collector circuit of the transistor 6 and the motor control winding 3, will cause unidirectional pulses of current to flow in the control winding which have an alternating current component in phase with the reference voltage. As `a result of :the inductiance of the control windin-g 3 and to some extent the action of the condenser 5, the wave form of the current in the control winding approaches `the form of a 60 cycle alternating current signal having aD. C. component. This 60 cycle current is in phase with the reference voltage and leads the current in the power winding by approximately 90 degrees, thus causing the motor 1 -to turn Iin the corresponding direction. The speed of this rotation is proportional to the magnitude of the control Winding current which lin turn is proportional to the magnitude of the control signal.

If the control signal lags the reference voltage by approximately 90 degrees, the voltage appearing across the emitter-collector circuit of `the transistor 6 and the motor control winding 3 causes pulses of unidirectional current to flow inthe motor control winding which have an alternating current component 180 degrees out rvof phase with the reference voltage. Due to the inductance l'of the motor control winding 3 Aand to some extent the l the opposite direction. Again, the speed of this rotation is proportional to .the magnitude of the control signal. As the control signal is reduced or disappears entirely, the inertia of the motor is sometimes suc-ient to maintain it in rotation -at a speed greater than that required Iby the output of the motor control circuit. When this happens, the motor acts as a generator, magnetically transferring electrical energy from the moto-r power winding 2 to the control winding 3, resulting in the appearance of a voltage across the winding 3. However, as the control signal is reduced, the impedance of the transistor 6 is reduced reaching a minimum when the control signal completely disappears. In this condition the transistor 6 4appears to be a -low impedance shunting the control winding 3. Due to this low impedance, the current which will ilow in the motor control winding, as a lresul-t of the volta-ge produced therein by the generator action of the coasting motor, will have such a phase as to set up a iiux field in the motor tending to drive it in a direction opposed to its rotation and thereby produce a braking action. In this respect, transistors, unlike vacuum tubes, can lbe made to exhibit extremely low impedfances. By a proper selection yof the transistor operating point, the 4impedance of the emitter-collector circuit of the transistor 6 can lbe made as low as a few ohms in the standby position thereby assuring good motor damping.

Referring now to Fig. 3 there is shown a modication of the circuit shown in Fig. l. Similar reference characters have been employed to designate corresponding 'elements and consequently these elements are not described in detail. The larrangement of the circuit of Fig. 3 differs from the arrangement of the circuit of IFig. l in that the motor control winding 3 is connected in yseries Iwith the condenser 41 across the output termin-als of the rectifier 9 and the emitter-collector circuit `of the transistor 6. The condenser 41 is lselected with respect to `the motor control winding 3 -so -as to `form therewith a substantially series resonant circuit. As shown in Fig. 2, this causes the current in the control winding 3 to be in phase with the voltage across the .series resonant :cir-cuit. Accordingly, it is necessary to shift the phase of the current in the power winding 2 to obtain motor operation. This is accomplished by means of the condenser 42 which is connected in series with the power Wind-ing 2 across the conductor L and L2. The condenser 42 is selected with respect to the power winding 2 so as to form therewith a substantially series resonant circuit.

The effect of the condenser 41 connected in series with the control winding 3 is twofold. It results lin an increase in motor power Iby improving 4the wave form of the current in the control winding. `On the other hand, it increases the impedance of the control winding .circuit and thus tends to reduce the d-amping qualities of the control circuit.

While, in accordance with the provisions of the statutes, there has |been illustrated and described the lbest Aform of the invention 'now' known, it will be apparent to th-ose skilled in the .art that changes may be made in lthe -form of the apparatus disclosed without departing from the spirit of the `invention as set forth in the appended claims, and that in some cases certain features of the invention may sometimes be used to advantage without a corresponding use of other features. It should also :be understood that while pnp junction transistors have ybeen illustrated in the drawings that npn junction transistors could Having now described this invention, what is claimed as new and for which it is desired to secure Letters Patent is:

l. In combination, a single :stage common emitter transistor amplifier, having an input and `an output, biasing means connected to the input of said amplifier for rendering it conductive in the absence of an input signal, a two phase reversible induction motor having a power winding and a control winding, said power winding being adapted to be connected to a source of alternating current, circuit means connecting said cont-rol winding in shunt with the output of said amplifier, vand a power supply, said power supply having an impedance of such magnitude with respect to the magnitude of said transistor and said motor control winding that changes in the impedance of said transistor will not produce appreciable changes in the current owin-g Vfrom said power supply.

2. In combination, .a two phase reversible induction motor having a power winding and a control winding, said power winding adapted to be connected to a source yof alternating current, a transistor amplifier for controlling the current in said control winding, said yamplifier comprising, a transistor vhaving an emitter, a collector and a base, a power supply, circuit means connecting the emitter and collector Iof said transistor to the power supply, an input circuit adapted to receive a control signal, circuit means connecting said input circuit to the base and emitter of said transistor, biasing means connected to said input circuit to render said transistor conductive in the absence of a control signal, and means connecting the motor control winding to the collector and the emitter of said transistor and in parallel therewith across said power supply and thus with said `transistor tending to draw therefrom a substantially constant current.

3. An electronic motor drive circuit .comprising in combination a two phase reversi-ble induction motor having a power `winding and a lcontrol winding, said power winding ybeing adapted to be 'connected `to a source of alternating current, an amplifier for supplying current to said control winding ycomprising a transistor having an emitter, a collector and a base, a power supply having an impedance substantially higher than that of `the transistor and the motor control Iwinding and thus tending to supply said transistor :and said motor control winding with a constant current, a circuit means connecting 'the emitter and collector of said transistor to -said power supply, an input circuit adapted to receive a phased input signal, circuit means connecting said input circuit to the base `and `emitter of said transistor, biasing means connected to said input circuit to render said transistor conductive in the absence of an input signal, and circuit means connecting said motor control winding to the emitter and collector of said transistor.

4. Apparatus as defined in claim 3 wherein said power supply comprises a bridge rectifier :adapted to be connected to a source of alternating current through a capaci tor having a high reactance at the operating frequency compared to the reactance of the transistor and motor control winding.

5. Apparatus as defined -in claim 3 wherein a capacitor is connected in parallel with said motor control winding to form therewith a substantially parallel resonant circuit.

6. Apparatus as defined in claim 3 wherein a capacitor is connected in series with said motor control winding to form therewith a substantially series resonant circuit and said motor power winding is connected to a source of alternating current through a capacitor forming therewith a substantially series resonant circuit.

7. In combination, a two phase rotating eld motor having a power winding and a control winding, said power winding being adapted to be connected to a source of alternating current power having a substantially fixed phase, a source of direct current, a transistor, means connecting said transistor and said control winding in parallel to said source `of direct current, and input signal means yfor rendering said transistor alternately conductive and non-conductive in relationship to the phase of said alternating current source.

8. Apparatus as defined in claim 7 wherein said direct current source comprises a bridge rectifier connected to said alternating current source by a capacitor and a resistor in series.

9. Apparatus as defined in claim 7 wherein said transistor has a direct current bias source connected thereto to maintain said transistor in a highly conductive state in the absence of an input signal on said input signal means.

10. Apparatus as defined in claim 7 wherein said input signal means comprises a transformer having a primary winding and a secondary winding, said primary Winding being adapted to be connected to a source Aof alternating input voltage having a reversible phase with respect to said alternating current power source, said secondary winding being connected to said transistor through a coupling condenser and lbiasing means comprising a resistor and a source of direct current connected in series across said transistor to render said transistor in a highly conductive state in the absence of an input signal.

11. In combination, a two phase rotating field motor having a power winding and a control winding, said power winding being adapted to be connected to a source of alternating current power having a substantially fixed phase, a pair of terminals adapted to be connected -to a source of direct current, a transistor, means connecting said transistor and said control winding in parallel to said pair of terminals, and input signal means for rendering said transistor alternately conductive and non-conductive in relationship to the phase of said alternating current source.

No references cited.